Fix `ForeverStack::find_starting_point` output parameter #8137

Job	Run time
build-and-check-gcc-48	28m 22s
build-and-check-ubuntu-32bit	25m 22s
build-and-check-clang-macos	55m 51s
build-and-check-asan	31m 21s
build-and-check-ubuntu-64bit	26m 40s
build-and-check-ubuntu-64bit-glibcxx	27m 34s
	3h 15m 10s

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